The present invention relates to a safety belt buckle and particularly to a buckle for use with a buckle end pretensioner.
Pretensioners are used to rapidly pull back the buckle mounting so as to rapidly take up any slack in the seat belt holding the vehicle occupant in position. Slack can arise if the occupant is out of position (for example asleep!) or from lose windings on the retractor (so called spool film effect). The buckle is pulled back very quickly and thus extremely high G-forces are exerted on the buckle parts. A conventional buckle tends to unlatch under the influence of these high G-forces because of relative motion between parts of the buckle due to their different inertias. For example some parts may lag behind the buckle frame during the accelerating phase of the pretensioning operation. Other parts keep moving after the frame of the buckle has been stopped (this is deceleration phase of the pretensioning operation). In particular, during the deceleration phase the actuating button of the buckle keep moving and acts in the same way as if was being depressed, to open the buckle. Obviously this is highly undesirable and it is an object to the present invention to provide a buckle which is immune to the high G-forces generated during pretensioning.
One known pretensioner proof buckle is disclosed in EP 681 792. Here a pin type catch is loaded by a compression spring to a retracted position during normal operation of the buckle. However under pretensioning the retraction force of the spring is overcome by the high G-forces during the acceleration phase and the pin is moved into a blocking position over the buckle latch. At the end of the pretensioning stroke, i.e. during the deceleration phase the button tries to move into a de-latching position and the force exerted by the button on the latch jams the pin in its blocking position. Thus theoretically the buckle does not open.
However this mechanism relies on carts of the buckle jamming together and this sometimes locks up the buckle totally so that it cannot be released after the pretensioning operation is complete. This is obviously highly undesirable since in that case the vehicle occupant cannot be released from the seat.
It is an object of the present invention to provide an improved pretensioner proof buckle which does not jam on use.
According to the present invention there is provided a buckle for a vehicle safety restraint belt comprising a latch moveable between a tongue encasing position in which the buckle is fastened and a tongue disengaged position in which the buckle is open, a button moveable in the direction of tongue insertion to lift the latch against the action of a biasing spring, into the disengaging position, a securing means mounted in the frame of the buckle to be moveable between a position in which it restrains a movement of the latch into the disengaging position and a position in which it does not restrain movement of the latch, the securing means being biased to the unrestraining position by resilient means, the strength of the resilient means being such as to be overcome during the deceleration phase of a pretensioner so that the securing means moves to the restraining position and prevents opening of the buckle and when pretensioning forces dissipate, the securing means moves back to the un-restraining position.
Preferably the biasing means is a leaf spring held at its ends in slots in the buckle frame and its centre in a slot or by a hook mounted on the securing means. The securing means is preferably a slidable plate mounted for a sliding movement in slots in the frame. The latch is preferably biased to the tongue engaging position by a leaf spring.
A buckle constructed according to the present invention has significant advantages over the prior art because the securing means moves into the latch restraining position during the deceleration phase of the pretensioning operation and thus the danger of the buckle jamming is significantly reduced.
Of course a coil spring may be used instead of a leaf spring to bias the securing means.
In the preferred embodiment in which the securing means is held in the frame c the buckle, this pretensioning overlocker is very strong and can withstand the extremely high loads that are applied to the button assembly under pretensioning. In one embodiment the securing means is made lighter than the button. The securing means moves faster than the button during the deceleration phase and blocks the latch before the button exerts opening forces on the latch. The ratio of the spring strength to the mass of the securing means controls this operation. Typically the securing means is only half the mass of the button and the spring is arranged so that the force needed to overcome the inertia of the button is a multiple, up to around nine times, the force needed to overcome the inertia of the securing means.
According to an alternative embodiment, the latch is allowed to travel a small distance under pretensioning conditions but is inhibited from lifting fully out of tongue engagement by a section in the lower frame by a spring loaded mass assembly behind the latch maintaining the position of the latch in normal conditions. When the arrangement is under load during pretensioning then the securing means is moved into position by a calibrated spring which is set so that blocking of the latch is achieved prior to the button moving.
According to yet another alternative embodiment the button is formed in two sections which move together under normal working conditions. However under pretensioning conditions the inner button is allowed to move prior to the outer button since it is made heavier and thus has higher inertia. The inner button has a small abutment which under pretensioning conditions fits over or clamps against the latch and prevents it from lifting and unlocking the buckle. Preferably this abutment comprises a composite material with a much higher friction coefficient than the normal material used for a button. The mass of the inner button and the value of the calibration spring counteract the forces apply by the outer button to try to lift the latch and disengage the buckle.